The present invention relates to a novel positive-working chemical-amplification photoresist composition capable of giving a very finely patterned resist layer having an excellently orthogonal cross sectional profile with high photosensitivity and pattern resolution on a substrate surface by patterning exposure using a KrF excimer laser beam and to a method for efficiently forming a very finely patterned resist layer of an isolated pattern having an excellently orthogonal cross sectional profile with fidelity to a photomask pattern by using the above mentioned photoresist composition.
As a trend in the manufacturing technology of semiconductor devices and liquid crystal display panels in recent years, extensive investigations are now under way to establish a photolithographic patterning process of a resist layer having a pattern resolution of as fine as 0.25 .mu.m or even finer by the use of a positive-working chemical-amplification photoresist composition. Turning now to the problem of the light source for the pattern-wise exposure of the photoresist layer to comply with the requirement in the manufacture of semiconductor devices to accomplish finer and finer patterning, a photolithographic patterning technology for obtaining a patterned resist layer of 0.15 to 0.22 .mu.m fineness by using a KrF excimer laser beam is the current target of the development works.
With an object to comply with the above mentioned requirements, a proposal is made in Japanese Patent Kokai 7-209868 for a positive-working chemical-amplification photoresist composition containing, as the film-forming resinous ingredient, a copolymeric resin consisting of hydroxyl group-containing styrene units, styrene units and tert-butyl (meth)acrylate units in a molar ratio of 40:20:40 or 33:17:50. The there proposed photoresist composition using a copolymeric resin with a relatively small amount of the hydroxyl group-containing styrene units or a relatively large amount of the tert-butyl (meth)acrylate units is not quite satisfactory when an extremely fine patterned resist layer with 0.15 to 0.22 .mu.m fineness is formed therewith because the cross sectional profile of the patterned resist layer is not fully orthogonal as desired.